Load bypass switching is a technique for placing a bypass switch in parallel with a load so that current (i.e., power) can be diverted around the load without interrupting the current. Slew rate is defined as the time rate of change of voltage across the bypass switch. A practical use of load bypass switching is pulse width modulation dimming of light-emitting diodes (LEDs). LEDs can be connected in series so that the same current flows in each LED, and thus, can ensure a matched light output. The light from an individual LED, or a multi-LED segment, can be extinguished for dimming purposes by actuating a switch in parallel with one or more of the LEDs to divert the current around that portion of the series-connected LEDs. A popular name for this technique is matrix LED dimming. Often, LED drivers regulate the current to the LEDs and not the voltage. When portions of the LED load are bypassed, the voltage across the overall load can change even as the current stays the same. Because the matrix dimming can generate voltage steps, there are situations where slew-rate control during the switch turn-on and turn-off transitions can ameliorate light intensity fluctuations that otherwise occur with the voltage steps. Slew-rate control can allow time to charge and discharge a storage capacitor across the LED string. For switch-mode DC/DC power converters that produce discontinuous current pulses, an output storage capacitor can filter or smooth current delivery to the load. A “boost” or step-up converter is a type of DC/DC converter that often uses an output storage capacitor.
Significant fluctuation of current in LEDs, as well as other types of loads, due to bypass switching events, can cause voltage or current spikes that can disrupt operation of the load, cause flicker in an LED load, or possibly damage the load due to electrical overstress. Conventional techniques to limit the voltage slew rate in bypass switching applications can add an integrating capacitor between a control node and a switched node (e.g., drain) of a bypass transistor. This capacitor is often referred to as a Miller capacitor. In addition, conventional techniques also charge or discharge the gate of the bypass transistor with a fixed current. When the bypass transistor is at the turn-on or turn-off threshold, current can begin to flow via the Miller capacitor to counteract the charging or discharging of the control node of the bypass transistor and thus provide a slew rate dependent on the capacitance value of the Miller capacitor.
The conventional techniques have a number of drawbacks. One drawback is that the slew rate is not adjustable once the Miller capacitor and the control node charging current are set. A second drawback is that the Miller capacitor adds to the overall capacitance on the control node, even when the control node is not at the turn-on or turn-off threshold and is not providing the benefit of bypass switch voltage slew-rate control. The additional capacitance can lead to unintentionally long delays in placing the control node at the threshold voltage when coupled with smaller charging and discharging currents. A third drawback is that an acceptable Miller capacitance to achieve adequate slew-rate control for a system including a boost-type switch mode converter can be impractically large to be an on-chip, integrated circuit element, or the charging/discharging current impractically small such that the charging/discharging current can be overwhelmed by junction leakage currents. Implementations of conventional slew rate techniques typically limit slew rates to 5V/μs for an integrated Miller capacitor. Slower slew rates often require use of a discrete external capacitor.